Nanocomposites are compositions that satisfy many of the challenges currently presented by automotive plastics and composites needs. These materials offer a variety of desirable properties including: low coefficient of thermal expansion, high heat distortion temperatures, lightweight, improved scratch resistance, and good surface appearance. Nanocomposite compositions are polymers reinforced with nanometer sized particles (“nanoparticles”), i.e., typically particles with a dimension on the order of 1 to several hundred nanometers. These materials can be used in structural, semi-structural, high heat underhood, and Class A automotive components, among others. In other words, these nanocomposites are compositions in which small particles are dispersed in the plastic matrix.
Injection moldable thermoplastics have long been mechanically reinforced with an addition of particulate and fiber fillers in order to improve mechanical properties such as stiffness, dimensional stability, and temperature resistance. Typical fillers include chopped glass fiber and talc, which are added at filler loadings of 20-40% in order to obtain significant mechanical reinforcement. At these loading levels, however, low temperature impact performance and material toughness are usually sacrificed. Polymer-silicate nanocomposite materials, in other words, compositions in which the silicate is dispersed as very small particles, can address these issues.
Polymer-layered silicate nanocomposites normally incorporate a layered to clay mineral filler in a polymer matrix. Layered silicates are made up of several hundred thin platelet layers stacked into an orderly packet known as a tactoid. Each of these platelets is characterized by large aspect ratio (diameter/thickness on the order of 100-1000). Accordingly, when the clay is dispersed homogeneously and exfoliated as individual platelets throughout the polymer matrix, dramatic increases in strength, flexural and Young's modulus, and heat distortion temperature are observed at very low filler loadings (<10% by weight) because of the large surface area contact between polymer and filler.
Clay minerals and their industrial applications are reviewed by H. M. Murray in Applied Clay Science 17(2000) 207-221. Two types of clay minerals are commonly used in nanocomposites: kaolin and smectite. The molecules of kaolin are arranged in two sheets or plates, one of silica and one of alumina. The most widely used smectites are sodium montmorillonite and calcium montmorillonite. Smectites are arranged in two silica sheets and one alumina sheet. The molecules of the montmorillonite clay minerals are less firmly linked together than those of the kaolin group and are thus further apart.
Polyamide nanocomposites typically combine a polyamide with an inorganic layered silicate, usually a smectite clay The alkali and alkaline earth ions in the layered silicate are exchanged with onium ions, typically alkyl ammonium ions from alkylammonium salts (for example octadecylammonium chloride or a quaternary ammonium tallow), or ω-amino acids (for example, 12-aminolauric acid) in order to facilitate intercalation and subsequent exfoliation. Clays that have been so treated are often referred to as “(organically) modified clays” or “organoclays.”However, these compounds are not thermally stable enough to be used with those polyamides that are compounded high temperatures, particularly semiaromatic polyamides.
Polyamide nanocomposites have been prepared via melt compounding (also referred to as “melt mixing”). In Japanese Patent Application H02[1990]-182758, Oda et al. melt compounded 15 and 30 wt % of sepiolite into polyamide 6 after drying the sepiolite for 24 h at 100° C. It describes the fiber diameter of the sepiolite as ordinarily about 0.05 to 0.3 μm, and the fiber length, about 1 to 100 μm. No particular restriction on the fiber diameter or the fiber length of the sepiolite is disclosed, but it is disclosed that sepiolite with a fiber diameter of about 0.1 to 0.2 μm and a fiber length of about 3 to 30 μm is easy to acquire and offers excellent results. It is also disclosed that the use of less than 5 wt % sepiolite does not achieve improvement in the properties of mechanical strength, heat resistance, and warpage.
For the reasons set forth above, there exists a need for improved polyamide nanocomposites with low concentrations of nanoparticles that can be processed at high temperatures and yield improved properties. The present invention fulfills such need.